This invention relates to the variable lift and duration mechanism (VLD) previously developed by the applicants of the present invention. It utilises two concentric camshafts the phase of which may be altered relative to one another. The purpose of these two camshafts is that the lift imparted to the valve is determined by the sum of the lift contributed by each camshaft profile. No lift is imparted to the valve when either camshaft is “off-cam”. By varying the phase of the two camshafts, the cumulative lift and duration can be altered. This results in directly altering the opening duration and lift of the engine valve, be it inlet or exhaust.
The cumulative lift is achieved by the use of a summation lever having cam followers in contact with both sets of cams. If either cam follower is on the base circle of the associated cam, the summation lever merely rocks about a pivot axis connecting it to a valve actuating rocker. If both cam followers are in contact with the cam lobes, the summation lever is displaced downwards, and pushes down on the actuating rocker which then pivots about a hydraulic lash adjuster to open the engine valve.
A fundamental aspect of the VLD system operation is that clearance must exist in the system when the valve(s) is closed and the rocker system moves through its ‘return’ or ‘reset’ motion. If the system were to be designed with no clearance, the effect of phasing the second cam lobe with respect to the first cam lobe to alter the main lift event would either introduce clearance, if the valve lift duration is increased, or cause an additional valve lift to occur during the return motion, if the valve lift duration is reduced.
Ensuring that the correct amount of clearance exists in the system is essential in order for the valve motion to correspond to the theoretical lift characteristic. Differing levels of clearance between engine cylinders will therefore manifest itself as valve lift variations between cylinders. This will cause different airflow through each cylinder of the engine, potentially causing misfires or poor engine stability.
Typically a shim surface, which contacts with the rotating portion of the camshaft, has been used to limit the expansion of the hydraulic lash adjuster (HLA) and therefore control this clearance. The clearance within each VLD rocker system can be set by either removing material from this shim surface or adjusting its location. Conventionally, a shim surface on the valve actuating rockers abuts with a collar on the side of the cam lobes to limit HLA inflation and therefore control clearance.
Using a surface on the camshaft to control clearance rather than a fixed stop surface attached to the cylinder head is advantageous because it prevents any variations in cylinder head geometry from affecting the operating clearance of the VLD rocker system. The clearance is defined only by the VLD rocker system and the camshaft, allowing any variations in all of the other components in the system to be compensated for by the HLA.
Whilst this design works very well to control clearance, the sliding interface between the shim surface and the camshaft collar results in a small frictional torque being applied to the camshaft that would not be present in a conventional valve train. There is also the potential for wear at this interface which would alter the clearance of the valve train and therefore potentially cause the valve to lift during the return motion of the rocker system.
Whilst such frictional losses from this interface are only considered minor, with strict fuel economy legislation and resulting efforts to improve engine efficiency it is desirable to eliminate any unnecessary frictional losses. This is particularly important for valve lift control systems which are primarily intended to deliver a fuel economy benefit.
It must also be noted that in common with all variable valve lift systems, the VLD rocker system has a number of additional component interfaces that would not be present in a conventional roller finger follower system. Whilst the VLD system will tend to have lower friction than a conventional valve train when it is running at its lower valve lift settings, at higher lifts the friction resulting from these additional interfaces becomes more significant. Furthermore, when operating at higher lifts the VLD rocker system will tend to increase the loads on the camshaft bearings, potentially increasing the camshaft frictional torque.